The present invention relates to the field of turbomachines such as gas turbine engine axial compressors and is aimed in particular at the machine's variable pitch stator vanes.
An articulated system, such as the variable pitch stator vanes of a gas turbine engine compressor, comprises parts which move relative to one another. FIG. 1 schematically depicts a variable pitch stator vane 1 mounted in the machine casing 3. The stator vane comprises an airfoil 12, a mounting plate or platform 13 and a pin forming a pivot 14 at one end. The pivot 14 is housed in a radial orifice or bore formed in the wall of the casing 3 via various bearings. The vane is held only at this end. The other end holds a floating annular element 16 in which it is pivotably mounted via a second pivot 17. The annulus is provided with sealing means to seal against the part of the rotor 18 adjacent to it.
The pivot 14 swivels in the corresponding bore of the casing via bearings, for example a bottom bearing 4 at the platform end. The platform 13 is housed in a cavity in the form of a counterbore machined in the wall of this casing. The wall of the casing is in radial contact with the platform 13 either directly or via a bushing or washer. The top part of the pivot 14 is held in a top bearing 5. The bearings 4 and 5 for example consist of bushings housed in the bore of the casing with an internal ring forming a rubbing surface to rub against the pin 14 that forms the pivot. A friction ring may also be shrunk onto the pin 14.
On the opposite side of the bearing 4 from the platform 13 is the base of the airfoil which is swept by the gases set in motion by the compressor. This face of the mounting plate is shaped to ensure the continuity of the string formed by the casing. A nut 15′ holds the vane in its housing and a lever actuated by appropriate control members, rotates the vane about the axis XX of the pin to place this vane in the required position with respect to the direction of the gas flow. The relative movements result from the sliding of surfaces in contact with one another.
In the case of a gas turbine engine axial compressor or alternatively in the case of an axial compressor of air or some other gas on its own, such as a blast furnace compressor or an actual gas compressor, the airfoil 12 is subjected over its entire length to the aerodynamic forces generated by the gas flow. The component of these forces that is directed at right angles to the chord in the intrados-to-extrados direction, generally passing through the axis of the pivot, is the largest component. It will also be noted that, when the vane is highly angled, the component may pass away from this axis. The airfoil is also subjected to axial static pressure forces in the upstream direction because of the pressure difference between the downstream and the upstream sides. The resultant force is illustrated by the arrow F in the figure. It then follows that a moment is applied which combines with the pitch-adjustment rotation about the axis XX over an amplitude which may reach and exceed 40 degrees.
As mentioned above, the bushings that are mounted under the levers of the variable pitch system permit no movement other than rotation about the axis of the pivot. However, because of the transverse forces and the flexing of the stators, the bushings of the prior art are found to exhibit uneven wear. Such wear is detrimental to the correct operation of the variable pitch device.